Which of these numbers is prime? ${35,\ 91,\ 93,\ 94,\ 97}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 35 are 1, 5, 7, and 35. The factors of 91 are 1, 7, 13, and 91. The factors of 93 are 1, 3, 31, and 93. The factors of 94 are 1, 2, 47, and 94. The factors of 97 are 1 and 97. Thus, 97 is a prime number.